Discoloration prevention in pyrithione-containing coating compositions

ABSTRACT

Disclosed is a process for inhibiting the formation of discoloration in an aqueous composition selected from the group consisting of water-based paint, adhesive, caulk and sealant compositions, and combinations thereof, wherein said discoloration is caused by the presence of ferric ion or cupric ion together with pyrithione in said composition. The process comprises contacting the composition with a discoloration-inhibiting amount of a zinc compound selected from the group consisting of zinc salts of organic acids, zinc salts of inorganic acids, zinc hydroxide, zinc oxide, and combinations thereof. Also disclosed is an aqueous antimicrobial composition protected against discoloration attributable to the presence of ferric ion or cupric ion therein.

FIELD OF THE INVENTION

This invention relates generally to coating compositions, and, morespecifically, to methodology for preventing or reducing discoloration inpyrithione-containing coating compositions.

BACKGROUND OF THE INVENTION

Sodium pyrithione (also called the sodium salt of1-hydroxy-2-pyridinethione, sodium pyridine-2-thiol-N-oxide, or2-pyridinethiol-1-oxide, Na salt) has excellent antimicrobialproperties, and is typically employed as a biocide and preservative infunctional fluids, such as metalworking fluids, lubricants, cosmeticsand toiletries. Sodium pyrithione is a well-known commercial productcommonly made by reacting 2-chloropyridine-N-oxide with NaSH and NaOH,as disclosed, for example, in U.S. Pat. No. 3,159,640.

Likewise, zinc pyrithione [also known as zinc pyridine-2-thiol-N-oxideor bis [1-hydroxy-2(H) pyridinethionato]-zinc] is an excellentantimicrobial additive. Zinc pyrithione may be made by reacting1-hydroxy-2-pyridinethione or a soluble salt thereof with a zinc salt(e.g., ZnSO₄) to form a zinc pyrithione precipitate, as disclosed, forexample in U.S. Pat. No. 2,809,971. Zinc pyrithione has been employed asa broad-spectrum anti-microbial agent and preservative in metalworkingfluids, plastics, paints, adhesives and cosmetics. Its principal usesare as an anti-dandruff agent in hair products, as a preservative invarious cosmetics, and as an antifoulant in marine paints. Thecommercial use of zinc pyrithione in paints, adhesives, caulks andsealants is growing.

In the presence of ferric ion, sodium or zinc pyrithione-containingcompositions tend to turn blue even though the ferric ion is present inmere trace amounts. This blue discoloration is undesirable for aestheticreasons, as well as for functional reasons relating to unwanted colorformulation.

More specifically, since the aesthetics of paints, adhesives, caulks andsealants normally require certain desirable colors, and since theformulators of such products go to great lengths to achieve specificcolor effects, any ingredient which causes the formulation to vary muchfrom a desired white or colorless (i.e., "water white") hue may make thecolorant formulators' task very difficult. More specifically, whenattempting to utilize pyrithione as an antimicrobial agent infully-formulated water-based paints, paint bases (i.e., the partiallyformulated paint before pigment addition), adhesives, caulks andsealants, an unwanted color in an additive can adversely affect thecolor of the formulated product. Thus, the discoloration typicallyadversely affects the desired color, producing an off-color product.

In addition to the aesthetics problems and colorant functioningproblems, the blue coloration problem associated with the presence offerric ion causes a performance problem in the sodiumpyrithione-containing compositions. This performance problem when usingsodium pyrithione results from the fact that the pyrithione tends toform a blue precipitate in the presence of ferric ion. The precipitatereduces the amount of available pyrithione throughout the composition,thereby diminishing the biocidal protection thereof.

In the past, various solutions to the blue discoloration problem havebeen proposed. By way of illustration, U.S. Pat. Nos. 4,957,658 and4,818,436 disclose solutions to the above-discussed discolorationproblem attributable to the presence of ferric ion plus pyrithione, inpaints and functional fluids (e.g., metalworking fluids) respectively,by adding to the paint or functional fluid an alkali metal or alkalineearth metal salt of 1-hydroxyethane-1,1-diphosphonic acid. Although the'658 and '436 patents propose good solutions to the discoloration, thesesolutions are not always as cost effective or permanent as might bedesired.

As another illustration, U.S. Pat. No. 4,161,526 discloses a white tocream yellow pyrithione, pyrithione salt or dipyrithione for applicationto skin or hair containing from about 0.01 percent to about 1 percent ofthe zinc salt of an organic carboxylic or inorganic acid, zinc hydroxideor zinc oxide, or a mixture thereof. The composition of the '526 patentis said to be effective in preventing or removing discoloration causedby formation of a colored pyrithione, pyrithione salt, or dipyrithionecontaminant (said to be iron pyrithione) in the composition.Unfortunately, the '526 patent does not teach a solution to thediscoloration problem in compositions unrelated to skin or hair care,and not containing iron pyrithione. Furthermore, the '526 patent teachesthe use of higher levels of the zinc salt, zinc oxide or zinc hydroxidethan otherwise might be desired.

New solutions to the blue and green discoloration problem in variousaqueous compositions, particularly paints, adhesives, caulks andsealants which enables pyrithione to be utilized therein in the presenceof iron or copper, without any resulting discoloration of thecomposition, and that is cheaper, longer lasting, and/or uses lowerlevels of additives than required by the above-discussed prior art,would be highly desired by the paint, adhesives, caulks and sealantsmanufacturing community. The present invention provides one suchsolution.

SUMMARY OF THE INVENTION

In one aspect, the present invention relates to a process for inhibitingthe formation of discoloration (e.g., a blue, grey, or green off-colorformation) in an aqueous composition selected from the group consistingof water-based paint, adhesive, caulk and sealant compositions, andcombinations thereof, wherein said discoloration is caused by thepresence of ferric ion or cupric ion together with pyrithione in saidcomposition, which comprises contacting the composition with adiscoloration-inhibiting amount (advantageously between 10 ppm and 90ppm, more advantageously between 10 ppm and 70 ppm, based upon theweight of the composition) of a zinc compound selected from the groupconsisting of zinc salts of organic acids (e.g., zinc acetate), zincsalts of inorganic acids (e.g., zinc sulfate and zinc chloride), zinchydroxide, zinc oxide, and combinations thereof.

In another aspect, the present invention relates to a process forremoving an undesirable discoloration in an aqueous antimicrobialcomposition containing a dissolved metal ion selected from the groupconsisting of ferric ion, cupric ion, and combinations thereof, andcontaining pyrithione (preferably in an antimicrobially effectiveamount), which comprises contacting said composition with a zinc ion ina molar amount at least equal to the amount of said dissolved metal ionin said composition. Advantageously, the zinc ion is present in thecomposition in an amount of between 10 ppm and 90 ppm, moreadvantageously between 10 ppm and 70 ppm, based upon the weight of thecomposition.

In yet another aspect, the present invention relates to an aqueousantimicrobial composition, protected against discoloration attributableto the presence of ferric ion or cupric ion therein, said compositionbeing selected from the group consisting of water-based paints,adhesives, caulks and sealants, and combinations thereof, saidcomposition comprising water, an organic base medium and zinc ion, saidzinc ion being present in said composition in an amount of between 10ppm and 90 ppm, more advantageously between 10 ppm and 70 ppm. The zincion is suitably provided in said composition by the incorporationtherein of a zinc salt of an organic acid or inorganic acid, zinchydroxide, zinc oxide, or a combination thereof in order to provide thedesired amount of zinc ion.

In still another aspect, the present invention relates to a coatedsubstrate comprising a substrate selected from the group consisting offerrous metal-containing substrates, copper-containing substrates, andcombinations, and a coating on said substrate, said coating comprisingpyrithione and a zinc compound selected from the group consisting ofzinc salts of organic acids (e.g., zinc acetate), zinc salts ofinorganic acids (e.g., zinc sulfate and zinc chloride), zinc hydroxide,zinc oxide, and combinations thereof. Advantageously, the zinc compoundis present in said coating in an amount of between 10 ppm and 90 ppm,more advantageously between 10 ppm and 70 ppm, based upon the weight ofthe coating.

These and other aspects will become apparent upon reading the followingdetailed description of the invention.

DETAILED DESCRIPTION OF THE INVENTION

It has now been surprisingly found in accordance with the presentinvention that the addition of zinc to a composition containingpyrithione and ferric or cupric ion reduces or avoids a discolorationproblem otherwise attributable to such composition. Without wishing tobe bound by any particular theory, the discoloration problem is believedto be caused by the formation of an insoluble iron pyrithioneprecipitate that not only discolors the composition, but also depletesthe available pyrithione in sodium pyrithione-containing compositions,thereby diminishing the antimicrobial efficacy of the composition.

The term "discoloration" as employed herein with respect topyrithione-containing compositions that also contain ferric ion orcupric ion dissolved therein is intended to describe any unacceptablegray, blue, black, purple, green, or color other than the natural coloror desired artificial color of the paint or paint base formulation.Starting materials employed in preparing the aqueous compositions of thepresent invention typically include tap water, as well as a source ofpyrithione in the form of sodium pyrithione, zinc pyrithione, andcombinations thereof. It is noted, for example, that the natural colorof sodium pyrithione itself is a clear yellow. It is quite common,however, for iron and/or copper contaminants to be introduced into theaqueous composition from the tap water used, causing discoloration ofthe composition. One way of quantifying the discoloration is bymeasuring the reflectance color parameters, and calculating a whitenessvalue from them. Another method is to visually inspect the compositionfor any signs of off-whiteness, as compared to the desired or whitecolor.

In water-based paints, adhesives, caulks and sealants a level of ferricor copper ion of 10 ppm or higher is not uncommon. By incorporating aneffective amount of the zinc salt of an organic acid or inorganic acid,zinc hydroxide or zinc oxide or a mixture thereof into the composition,the blue discoloration typically attributable to the presence of ferricion bound with pyrithione is suitably reduced, eliminated or avoided, asis the green discoloration attributable to the presence of copper ion.

Although the amount of the above described zinc salt of an organic acidor inorganic acid, zinc hydroxide or zinc oxide, or combination thereof,needed to prevent discoloration in the aqueous composition in which itis employed can vary over a wide range of between 0.001% or lower to 10%or greater, based upon the weight of the aqueous composition in which itis employed, it has been surprisingly found in accordance with thepresent invention that the discoloration associated with tap watercontamination by iron or copper is suitably reduced or avoided byincorporating into said composition an amount of between 10 ppm and 90ppm, more advantageously between 10 ppm and 70 ppm, of the zinc ion.Although not wishing to be bound by any particular theory, the presentinventors believe that the efficacy of such low levels of zinc ion inresolving the discoloration problem is attributable to a "common ioneffect" forcing an equilibrium toward unionized zinc pyrithione wherezinc ion, pyrithione ion, and unionized zinc pyrithione are all presentin the composition.

The pyrithione used in the process and composition of this invention ispreferably a pyrithione salt, such as sodium pyrithione, zincpyrithione, chitosan pyrithione, magnesium disulfide pyrithione, and thelike, although pyrithione acid can be used if desired. More preferablepyrithione salts include sodium pyrithione, and zinc pyrithione, mostpreferably zinc pyrithione.

The sodium pyrithione useful in the present invention is a well-knowncommercial product that is commonly made by reacting2-chloropyridine-N-oxide with NaSH and NaOH, as illustrated by thedisclosures of U.S. Pat. No. 3,159,640.

Zinc pyrithione may be made by reacting 1-hydroxy-2-pyridinethione(i.e., pyrithione acid) or a soluble salt thereof with a zinc salt(e.g., ZnSO₄) to form a zinc pyrithione precipitate, as illustrated bythe disclosures of U.S. Pat. No. 2,809,971.

The aqueous compositions of the present invention are suitable for avariety of uses, such as, for example as soap, shampoo, skin caremedicaments, paint, or incorporated into or onto plastic or a woven ornon-woven fibers, when formulated to contain the requisite components inaddition to the antimicrobial component.

The antimicrobial compositions of the present invention are particularlyuseful in the form of paints, including indoor and outdoor householdpaints, industrial and commercial paints. Particularly advantageousresults are obtained when the compositions of the present invention areutilized as marine paints for use, for example, on ship's hulls. Inaddition, the antimicrobial compositions provide desirable results whenthe antimicrobial component is incorporated into exterior paints of thelatex and alkyd types. The antimicrobial component of the aqueouscomposition is also useful as an "in-can" preservative during storageand prior to use of the paint.

Typically a paint composition will contain, in addition to theantimicrobial component, a resin, a pigment, and various optionaladditives such as thickening agent(s), wetting agents and the like, asis well known in the art. The resin is preferably selected from thegroup consisting of vinyl, alkyd, epoxy, acrylic, polyurethane andpolyester resins, and combinations of thereof. The resin is preferablyemployed in an amount of between about 20% and about 80% based upon theweight of the paint or paint base.

In addition, the paint composition of the present invention optionallyadditionally contains optional additives which have a favorableinfluence on the viscosity, the wetting power and the dispersibility, aswell as on the stability to freezing and electrolytes and on the foamingproperties. If a marine paint is being fabricated, the paint preferablycontains a swelling agent to cause the paint to gradually "slough off"in its marine environment, thereby causing renewed biocidal efficacy ofnewly exposed biocide at the surface of the paint in contact with thewater medium of the marine environment. Illustrative swelling agents arenaturally occurring or synthetic clays, such as kaolin,montomorillonite, and bentonite), clay mica (muscovite), and chlorite(hectonite), and the like. In addition to clays other swelling agents,including natural or synthetic polymers, such as that commerciallyavailable as POLYMERGEL, have been found to be useful in thecompositions of the present invention to provide the desired "sloughingoff" effect. Swelling agents can be used singly or in combination. Thetotal amount of optional additives is preferably no greater than 20% byweight, more preferably between about 1% and about 5% by weight, basedupon the total weight of the paint composition.

Illustrative, thickening agents include cellulose 30 derivatives, forexample methyl, hydroxyethyl, hydroxypropyl and carboxymethyl cellulose,poly(vinyl alcohol), poly (vinylpyrolidone), poly(ethyleneglycol), saltsof poly(acrylic acid) and salts of acrylic acid/acrylamide copolymers.

Suitable wetting and dispersing agents include sodium polyphosphate,salts of low molecular weight poly(acrylic acid), salts of poly(ethanesulfonic acid), salts of poly (vinyl phosphonic acid), salts ofpoly(maleic acid) and salts of copolymers of maleic acid with ethylene,1 olefins 3 to 18 carbon atoms and/or styrene.

In order to increase the stability to freezing and electrolytes theremay be added to the paint composition various monomer 1.2-diols forexample glycol, propylene glycol (1.2) and butylene glycol 1.2) orpolymers thereof, or ethoxylated compounds. For example reactionproducts of ethylene oxide with long-chain alkanols, amines, alkydphenols, poly(propyleneglycol), or poly(butylene glycol), or acombination thereof, or the like.

The minimum temperature of film formation (white point) of the paintcomposition may be reduced by adding solvents, such as ethylene glycol,butyl glycol, ethyl glycol acetate, ethyl diglycol acetate, butyldiglycol acetate, benzene or alkylated aromatic hydrocarbons. Asdefoaming agents there are suitable for example poly(propylene glycol)and polysiloxanes. Optionally other biocides can additionally beincorporated into the paint formulations of the present invention.Useful optional solvents include methylisobutylketone (herein referredto as "MIBK"), xylene, ethyl benzene, methanol, and combinationsthereof.

The paint composition of the present invention may be used as a paintfor natural or synthetic materials, for example wood, paper, metals,textiles and plastics. It is particularly suitable as an outdoor paint,and is excellent for use as a marine paint.

Another significant use for the aqueous composition of the presentinvention is as a latex tile adhesive typically containing, for example,in addition to the antimicrobial component, a latex emulsion, anoptional rosin emulsion, an optional plasticizer, an optionalantioxidant, and an optional pigment or filler (such as calciumcarbonate). Yet another significant use for the aqueous composition ofthe present invention is as a latex caulk, typically containing, inaddition to the antimicrobial component, an acrylic latex, a nonionicsurfactant, a dispersant, an optional plasticizer, and an optionalpigment or filler (such as calcium carbonate).

The aqueous antimicrobial compositions of the present invention areuseful, in any of the variety of applications described herein, asdisinfectants and preservatives, in a liquid or spreadable solid form,alone or in combination with an inert carrier such as water, liquidhydrocarbons, ethanol, isopropanol, or the like. They can be employedusing conventional procedures to control bacteria and fungi in varioussubstrates, and can be applied to bacterial or fungal organisms or theirsubstrates in an antimicrobial amount by conventional procedures such asspraying, dipping, drenching impregnation, and the like.

The invention is further illustrated by the following Examples. Unlessotherwise stated, the "parts" and "%" are "parts by weight" and "percentby weight", respectively.

While the invention has been described above with references to specificembodiments thereof, it is apparent that many changes, modifications andvariations can be made without departing from the inventive conceptdisclosed herein. Accordingly, it is intended to embrace all suchchanges, modifications and variations that fall within the spirit andbroad scope of the appended claims. All patent applications, patents andother publications cited herein are incorporated by reference in theirentirety.

The following examples are intended to illustrate, but in no way limitthe scope of, the present invention.

EXAMPLE 1 Elimination of Blue Discoloration Caused by the Presence ofPyrithione and Ferric Ion in a Paint Base

Efficacy of zinc oxide in eliminating blue coloration caused by thepresence of ferric ion in sodium pyrithione containing paint bases.

In order to test the effectiveness of zinc oxide in eliminating orreducing blue color caused by ferric ion in a pyrithione-containingpaint, the following experiment was conducted.

Two samples weighing 150 grams each of below aqueous (latex) paintformulation containing sodium pyrithione and zinc oxide were placed inplastic cups. A wooden tongue depressor was then dipped into each sampleand then allowed to dry to provide a control or "blank" comparison.Ferric chloride was then added to each sample to provide a concentrationof 64 ppm of ferric ion in each sample. Tongue depressor coatings weretaken to provide a basis for comparison. No discoloration formed in thepaints themselves or the dry films of the painted tongue depressor.

    ______________________________________                                        Latex Paint Formulation Containing Sodium Pyrithione                          and Zinc Oxide:                                                               Ingredient             Grams                                                  ______________________________________                                        water                  240.00                                                 hydroxyethyl cellulose 6.0                                                    Tamol 850.sup.1/       14.2                                                   Ethylene Glycol        50.0                                                   Colloid 643.sup.2/     2.0                                                    Triton ® CF-10.sup.3/                                                                            5.0                                                    sodium pyrithione 40% active                                                                         3.0                                                    potassium tripolyphosphate                                                                           3.0                                                    Pigment grind:                                                                titanium dioxide (Rutile)                                                                            424.0                                                  aluminum magnesium silicate                                                                          228.0                                                  attapulgite clay       3.0                                                    zinc oxide             50.0                                                   aluminum silicate      100.0                                                  propylene glycol       68.0                                                   Let Down:                                                                     water                  84.0                                                   acrylic latex emulsion 58.0% solids                                                                  700.0                                                  Colloid 643            6.0                                                    Texanol ®.sup.4/   18.6                                                   hydroxyethyl cellulose 2.5% in water                                                                 236.4                                                  Total Mass in Grams    2243.2                                                 ______________________________________                                         .sup.1 An anionic dispersant, a product of Rohm and Haas Company              .sup.2 A defoamer, a product of RhonePoulence Corp.                           .sup.3 A nonionic surfactant, a product of Union Carbide Corp.                .sup.4 A coalescent, a product of Eastman Kodak Company                  

Next, as a comparison, two samples weighing 150 grams each of belowaqueous (latex) paint formulation containing sodium Pyrithione and nozinc oxide were placed in paper cups. A wooden tongue depressor was thendipped into each sample and then allowed to dry to provide a control or"blank" comparison. Ferric chloride was then added to each sample toprovide a concentration of 64 ppm of ferric ion in each sample. Thecomparison coatings on the tongue depressor were visually observed toprovide a basis for the comparison. After 30 minutes a bluish graydiscoloration formed in the paints themselves and the dry films of thepainted tongue depressor.

    ______________________________________                                        Latex Paint Formulation Containing Sodium Pyrithione                          and Zinc Oxide:                                                               Ingredient             Grams                                                  ______________________________________                                        water                  240.00                                                 hydroxyethyl cellulose 6.0                                                    Tamol 850.sup.5/       14.2                                                   Ethylene Glycol        50.0                                                   Colloid 643.sup.6/     2.0                                                    Triton ® CF-10.sup.7/                                                                            5.0                                                    sodium pyrithione 40% active                                                                         3.0                                                    potassium tripolyphosphate                                                                           3.0                                                    Pigment grind:                                                                titanium dioxide (Rutile)                                                                            424.0                                                  aluminum magnesium silicate                                                                          228.0                                                  attapulgite clay       3.0                                                    aluminum silicate      100.0                                                  propylene glycol       68.0                                                   Let Down:                                                                     water                  84.0                                                   acrylic latex emulsion 58.0% solids                                                                  700.0                                                  Colloid 643            6.0                                                    Texanol ®.sup.8/   18.6                                                   hydroxyethyl cellulose 2.5% in water                                                                 236.4                                                  Total Mass in Grams    2193.2                                                 ______________________________________                                         .sup.5 An anionic dispersant, a product of Rohm and Haas Company              .sup.6 A defoamer, a product of RhonePoulence Corp.                           .sup.7 A nonionic surfactant, a product of Union Carbide Corp.                .sup.8 A coalescent, a product of Eastman Kodak Company                  

EXAMPLE 2 Elimination of Blue Discoloration Caused by the Presence ofPyrithione and Ferric Ion in a Paint

Efficacy of zinc oxide in eliminating blue coloration caused by thepresence of ferric ion in zinc pyrithione containing paint.

In the presence of ferric ion, zinc pyrithione-containing paintcompositions also tended to turn blue to gray, although at a much slowerrate than did the sodium pyrithione-containing paints.

48 percent aqueous zinc pyrithione was added to a white paint whichcontained 1.0 percent zinc oxide to provide a level of 3000 ppm of zincpyrithione in the sample. Ferric chloride was then added to provide aconcentration of 64 ppm of ferric ion in the sample. Upon standing for amonth, the paint was found to have no discoloration.

When this composition is painted over bare steel or copper, nodiscoloration is detected. This result is surprising, since the copperwould have been expected to turn the paint a greenish color, and thesteel would have been expected to turn the paint a bluish color.

As a comparison, 48 percent aqueous zinc pyrithione is added to a whitepaint containing no zinc oxide to provide a level of 3000 ppm of zincpyrithione in the sample. Ferric chloride is then added to provide aconcentration of 64 ppm of ferric ion in the sample. Upon standing for aweek, the paint is found to turn bluish in color.

EXAMPLE 3 Process for the Removal of Discoloration Caused by thePresence of Pyrithione and Ferric Ion in Water-based Coatings

48 percent aqueous zinc pyrithione was added to a white paint sample toprovide a level of 3000 ppm of zinc pyrithione in the sample. Ferricchloride was then added to provide a concentration of 25 ppm of ferricion in the sample. Upon standing for two days, the paint was found toturn bluish in color. To this point 0.007 percent (70 ppm) of zincsulfate was added and the paint was mixed for 5 minutes. After sittingfor an additional 5 minutes it was observed that the paint had whitenedand the bluish color was no longer noticeable. The addition of the zincsulfate to this paint had removed the blue color which was formed fromthe addition of zinc pyrithione and ferric chloride.

What is claimed is:
 1. A coated substrate comprising a substrateselected from the group consisting of ferrous metal-containingsubstrates, copper-containing substrates, and combinations, and adiscoloration resistant coating on said substrate, said coatingcomprising pyrithione and a zinc compound selected from the groupconsisting of zinc salts of organic acids, zinc salts of inorganicacids, zinc hydroxide, zinc oxide, and combinations thereof, said zinccompound being present in said coating in an amount of between 10 ppmand 90 ppm, based upon the weight of the coating.
 2. The coatedsubstrate of claim 1 wherein said zinc compound is present in saidcoating in an amount of between 10 ppm and 70 ppm based upon the weightof the coating.
 3. A coated substrate comprising a metal substrate and adiscoloration resistant coating on said substrate, said coatingcomprising pyrithione and a zinc compound selected from the groupconsisting of zinc salts of organic acids, zinc salts of inorganicacids, zinc hydroxide, zinc oxide, and combinations thereof, said zinccompound being present in said coating in an amount of between 10 ppmand 90 ppm, based upon the weight of the coating.
 4. The coatedsubstrate of claim 3 wherein said zinc compound is present in saidcoating in an amount of between 10 ppm and 70 ppm based upon the weightof the coating.